Civil Engineering Reference
In-Depth Information
2000
Table 8-1. Effect of Entrained Air on Concrete
Properties
1800
Properties
Effect
Abrasion
Little effect; increased strength increases
abrasion resistance
1600
Absorption
Little effect
1400
Alkali-silica reactivity
Expansion decreases with increased air
Bleeding
Reduced significantly
1200
Bond to steel
Decreased
Compressive strength
Reduced approximately 2% to 6% per per-
centage point increase in air; harsh or lean
mixes may gain strength
1000
Creep
Little effect
800
Deicer scaling
Significantly reduced
Density
Decreases with increased air
600
Fatigue
Little effect
Finishability
Reduced due to increased cohesion (sticki-
ness)
400
Flexural strength
Reduced approximately 2% to 4% per per-
centage point increase in air
Symbols:
Non-air-entrained
Air-entrained
200
Freeze-thaw
Significantly improved resistance to
resistance
water-saturated freeze-thaw deterioration
0
Heat of hydration
No significant effect
0
1
2
3
4
5
6
Modulus of elasticity
Decreases with increased air
Air content, percent
(static)
approximately 720 to 1380 MPa (105,000
to 200,000 psi) per percentage point of air
Fig. 8-2. Effect of entrained air on the resistance of concrete to
freezing and thawing in laboratory tests. Concretes were made
with cements of different fineness and composition and with
various cement contents and water-cement ratios (
Bates and
others 1952
, and
Lerch 1960
).
Permeability
Little effect; reduced water-cement ratio
reduces permeability
Scaling
Significantly reduced
Shrinkage (drying)
Little effect
Slump
Increases with increased air approximately
25 mm (1 in.) per
1
⁄
2
to 1 percentage point
increase in air
aggregate eventually cause significant expansion and
deterioration of the concrete. Deterioration is visible in the
form of cracking, scaling, and crumbling (Fig. 8-3).
Powers
(1965)
and
Pigeon and Pleau (1995)
extensively review the
mechanisms of frost action.
Hydraulic pressures are caused by the 9% expansion
of water upon freezing; in this process growing ice crystals
displace unfrozen water. If a capillary is above critical sat-
uration (91.7% filled with water), hydraulic pressures
result as freezing progresses. At lower water contents, no
hydraulic pressure should exist.
Osmotic pressures develop from differential concen-
trations of alkali solutions in the paste (
Powers 1965a
). As
pure water freezes, the alkali concentration increases in
the adjacent unfrozen water. A high-alkali solution,
through the mechanism of osmosis, draws water from
lower-alkali solutions in the pores. This osmotic transfer
of water continues until equilibrium in the fluids' alkali
concentration is achieved. Osmotic pressure is considered
a minor factor, if present at all, in aggregate frost action,
whereas it may be dominant in certain cement pastes.
Osmotic pressures, as described above, are considered to
be a major factor in “salt scaling.”
Capillary ice (or any ice in large voids or cracks)
draws water from pores to advance its growth. Also, since
most pores in cement paste and some aggregates are too
Specific heat
No effect
Sulfate resistance
Significantly improved
Stickiness
Increased cohesion—harder to finish
Temperature of wet
No effect
concrete
Thermal conductivity
Decreases 1% to 3% per percentage point
increase in air
Thermal diffusivity
Decreases about 1.6% per percentage
point increase in air
Water demand of wet
Decreases with increased air;
concrete for equal
approximately 3 to 6 kg/m
3
(5 to 10
slump
lb/yd
3
) per percentage point of air
Watertightness
Increases slightly; reduced water-cement
ratio increases watertightness
Workability
Increases with increased air
Note: The table information may not apply to all situations.
ment in durability effected by air entrainment is shown in
Figs. 8-2 and 8-3.
As the water in moist concrete freezes, it produces
osmotic and hydraulic pressures in the capillaries and
pores of the cement paste and aggregate. If the pressure
exceeds the tensile strength of the paste or aggregate, the
cavity will dilate and rupture. The accumulative effect of
successive freeze-thaw cycles and disruption of paste and
